Studies On The Co-catalyst Mechanism Of Metal Oxides In CO And Methanol Electro-oxidation On Pt

At present the research on the metal oxides which are used as carriers or cocatalysts of anode catalysts of fuel cells is still in the early stage. There is not an acceptable mechanism to explain how the metal oxides enhance the kinetics of methanol and carbon monoxide electrooxidation. This paper primarily studies the promoting effect of the structure and the category of the metal oxide to the electroxidation of methanol and carbon monoxide on Pt. This research provides theoretical and experimental evidences for designing new catalysts.In this paper, carbon nanotubes coated with metal oxides were used as the carriers. Then Pt nanoparticles were loaded on the carriers. The prepared Pt/MO₂/CNT catalysts (M = Ti, Ce, Sn) were characterized by physical methods. Pt nanoparticles are almost the same size on different catalysts and contact well with metal oxides, which is in favor of the synergistic effect. In acid solution, the activity of methanol electrooxidation on these catalysts is remarkably enhanced. The impedance spectrums of methanol oxidation indicate that the poisoning species of methanol electrooxidation can be oxidized at lower potentials on Pt/MO₂/CNT electrodes than that on Pt/CNT electrode. The required oxygen species are provided by metal oxides.The addition of metal oxides can accelate the production rate of hydroxyl groups on Pt/MO₂/CNT electrodes, which makes the onset potential and the peak potential of CO electrooxidation shift to lower potentials. According to the shape of CO electrooxidation peak, we think that at low potentials CO_ads react with the hydroxyl groups provided by the metal oxides. This reaction occurs at the interface of Pt nanoparticles and the metal oxides. Additionally, the rate of this reation is changed with the diffusion rate of CO_ads. At high potentials, hydroxyl groups are largely produced on Pt nanoparticles, which block the CO_ads diffusion on Pt, so CO_ads mainly react with the hydroxyl groups on Pt.Combined the results of electrochemical experiments with the characters of oxygen-containing species of different metal oxides and the Pt/MO₂/CNT catalysts, we think that the peak potential of the reaction of CO_ads and MO₂-OH_ads depends on the basic strength of metal oxides. And the CO electrooxidation charge of the reaction of CO_ads and MO₂-OH_ads depends on the production rate and quantity of hydroxyl groups provided by the metal oxides.The stability of Pt/MO₂/CNT catalysts in acid solution was tested by electrochemical methods. The anchoring function of metal oxides can enhance the stability of Pt nanoparticles. At the same time, metal oxides dissolve in the acid solution which reconstructs their morphology on CNTs. The dissolved metal ions interact with the Pt electrode, which can also enhance the activity of CO elelctrooxidation. The cocatalysis effect is decided by the characters of metal ions.